1. Field of the Invention
The present invention relates to a surface active agent for emulsion fuels. More particularly, the present invention relates to a surface active agent which is suitably used when a water-in-oil type emulsion fuel is prepared by adding water and a surface active agent to a fuel oil such as kerosine or heavy oil for the purpose of reducing the concentration of nitrogen oxides (NO.sub.x) in an exhaust gas discharged from a boiler, a heating furnace or the like.
2. Description of Prior Arts
It is known that NO.sub.x and other pollutants generated by combustion of various fuel oils cause air pollution. As means for preventing occurrence of this undesirable phenomenon, there have been proposed various methods for reducing the amount of NO.sub.x discharged into the open air. These methods are roughly divided into two types. According to the method of one type, generation of NO.sub.x is controlled, and according to the method of the other type, generated NO.sub.x is removed (called "denitration method"). According to one embodiment of the first-mentioned type of known methods for controlling generation of NO.sub.x, the structure of a burner or combustion chamber is improved or the combustion exhaust gases are recycled, namely the combustion system per se is improved. According to a second embodiment of the first-mentioned type of known methods for controlling generation of NO.sub.x, the kind of fuel to be used is changed or water or other additive is added to the fuel prior to or at combustion, namely the fuel per se is improved.
NO.sub.x is a term used to indicate both NO and NO.sub.2, and it is said that about 95% of NO.sub.x in a boiler exhaust gas is NO. NO.sub.x in the exhaust gas is divided into NO.sub.x generated from nitrogen-containing components contained in fuel oil, such as pyridine, pyrrole, quinoline and the like (called "fuel NO.sub.x "), and NO.sub.x generated by reaction between N.sub.2 and O.sub.2 in air at combustion (called "thermal NO.sub.x "). The concentrations of both the fuel NO.sub.x and the thermal NO.sub.x are increased as the temperature is elevated, and it is known that the concentration of thermal NO.sub.x is increased especially prominently at temperatures higher than 1500.degree.-1600.degree. C.
In the case of a water-in-oil type emulsion fuel formed by emulsifying water in the liquid fuel by a surface active agent, it is said that the amount of NO.sub.x generated by combustion is controlled according to the following three principles.
(1) Generation of NO.sub.x is greatly influenced by the combustion temperature and it can be controlled by lowering of the combustion temperature. In the case of an emulsion fuel, the combustion temperature is lowered by evaporation of water and generation of NO.sub.x is controlled.
(2) Fine fuel particles are made much finer owing to an abrupt expansion of the volume caused by evaporation of the water drops dispersed in an emulsion fuel. Accordingly, the contact area between the oil drops and air is increased and the ratio of air/fuel oil (kerosine or heavy oil) can be reduced, and the use of a low excess of air for combustion becomes possible. As a result, generation of NO.sub.x can be controlled. In general, the air/fuel ratio is brought close to 1, the concentrations of excess N.sub.2 and O.sub.2 at combustion are reduced and hence, the speed of formation of NO.sub.x (especially thermal NO.sub.x) is lowered and generation of NO.sub.x can be controlled.
(3) In an emulsion fuel, water is uniformly dispersed (emulsified), and when water is evaporated, the flame surface temperature is lowered by latent heat of water and simultaneously, local temperature elevation is prevented. Further, since fuel particles are made much finer in the emulsion fuel as pointed out in (2) above, local elevation of the temperature is prevented. As a result, generation of NO.sub.x can be controlled.
As the fuel formed by incorporating water into fuel oil, in addition to the above-mentioned emulsion oil, there can be mentioned a fuel formed by jetting water vapor into fuel oil and a fuel formed by jetting water into fuel oil. When these fuels are used or combustion methods using these fuels are adopted, the effect of controlling generation of NO.sub.x is considerably lower than the effect attained by an emulsion fuel. The reason is considered to be that in these fuels, water drops are not uniformly dispersed (emulsified) in the fuel oil or the particle size of the water drops is large. Further, these fuels are inferior with respect to the combustion stability.
In general, emulsions are evaluated based on the ease in emulsification and the stability of the emulsion. Emulsion fuels are burnt just after preparation thereof. Accordingly, in the case of emulsion fuels, ease in emulsification is important and a stability such that the emulsion is stable for 5 to 10 minutes or about 30 minutes at longest is sufficient for ordinary application.
When emulsion fuels are prepared, a surface active agent (surfactant) is ordinarily used in an amount of 1 to 10% by weight based on the total system (oil+water). When an emulsion fuel formed by using an appropriate commercially available surfactant, for example, polyoxyethylene nonylphenyl ether, polyoxyethylene dodecyl ether or polyoxyethylene sorbitan laurate in an amount of 1 to 10% by weight (based on the total system) is used as a fuel for a boiler, the concentration of NO.sub.x generated is much reduced as compared with the NO.sub.x concentration observed when the fuel oil alone is used. However, an emulsion fuel prepared by using such a large amount of a surfactant is very expensive and from the economical viewpoint, such an emulsion fuel is not hardly put into practical use.